NMR is a common laboratory and field tool used throughout many industries. One example is the oil industry which uses NMR to analyze the content and environment of water, oil, and gas in porous media like subterranean reservoir rock.
In general, NMR detects the amount of hydrogen (for proton NMR) in a sample or object under study. The lifetime of the detected NMR signal depends on the environment of the hydrogen. For example, an NMR signal detected from hydrogen in oil generally decays faster than an NMR signal detected from hydrogen in free water. In oil field application, differences in the rate of decay of NMR signals can be used to distinguish among for example, oil, water and gas in rock.
NMR signal decay times can also be used to distinguish among clay bound (“CBW”), capillary bound (“BVI”) and free fluid (“FFI”) quantities in reservoir rock.
A common method to determine CBW, BV1 and FFI quantities is to use either T2 or T1 relaxation times. The NMR relaxation parameters T1 and T2 are known by those skilled in the art to follow the following equations:
                              1                      T            2                          =                              1                          T                              2                -                Bulk                                              +                      ρ            ⁢                          S              V                                +                                                    (                                  γ                  ⁢                                                                          ⁢                                      GT                    E                                                  )                            2                        ⁢                                          D                o                            12                                                          (        1        )                                          1                      T            1                          =                              1                          T                              1                -                Bulk                                              +                      ρ            ⁢                          S              V                                                          (        2        )            where ρ is the relaxometry constant, Do is the free diffusion constant for the fluid, γ is the gyro magnetic ratio, G is the internal field gradient, TE is the echo time (a measurement parameter), and SN is the surface to volume ratio of the pores.
Equations (1) and (2) above reduce to direct relationships to the surface to volume ratio due to the fact that, in rocks, the bulk relaxation times are much longer than the measured values and typically, an echo time (TE) is selected such that the diffusion term can be ignored. The surface to volume ratio is a measure of the pore size distribution of the rock being studied.
The relaxation parameters T1 and T2 can be measured using many different NMR measurement pulse sequences known to those skilled in the art. For example, for T2 analysis, a prior art CPMG sequence can be used (see FIG. 1) and for T1 analysis, a prior art inversion recovery sequence can be used (see FIG. 2), where RF is the radio frequency excitation pulses, Tau is a delay time equal to ½ the echo time (TE) and ACQ is acquired NMR signal.
A problem with the above method is that if there is more than one fluid in the porous media being analysed (e.g. subterranean reservoir rock), the results no longer follow the simple equations 1) and 2) above. This makes the results difficult, if not impossible, to interpret.